With a grinding tool in which a rotation shaft of a grinding disc is rotatably attached at a position that is eccentric with respect to the axis of a drive shaft of a driving motor, the grinding disc performs an orbital motion around the drive shaft and a rotational motion about the rotation shaft. Therefore, such a grinding tool is usually called a “double action sander grinding tool” or a “random action grinding tool”. FIG. 1 illustrates an eccentric rotation mechanism of such an existing grinding tool. In a grinding tool 1, an eccentric rotation shaft 4 of a grinding disc 3, to which an abrasive member 2 is attached, is rotatably attached to a rotary disc 7 through a bearing 9 at a position that is eccentric with respect to an axis 6 of a drive shaft 5, which is connected to a motor not shown), and the rotary disk 7 is fixed to the drive shaft 5. Therefore, the grinding tool 1 performs grinding as the grinding disc 3 performs an irregular and complex rotational motion. Therefore, not only grinding can be performed efficiently but also generation of conspicuous marks or patterns, which are called “aurora marks”, can be prevented. Such marks and patterns are generated when a surface is ground by using an ordinary grinding tool that performs regular rotational motion, and they are observed when the surface, which appears to be smooth, is irradiated with light at a certain angle. Such marks and the like are generated because of very small and cyclical irregularities on the ground surface caused by regular rotation. It is possible to solve such a problem by using a grinding disc that performs an irregular rotational motion.
With the grinding tool 1, when the motor rotates the drive shaft 5, the rotary disc 7 rotates, and the grinding disc 3 performs an orbital motion, having an eccentric amount a as the radius, around the axis 6 of the drive shaft 5. The grinding disc 3 is rotatably attached to the rotary disc 7 through the eccentric rotation shaft 4 and the bearing 9. As the rotary disc 7 rotates, the grinding disc 3 rotates about an axis 8 of the eccentric rotation shaft 4 due to a driving force generated by friction between the eccentric rotation shaft 4 and the bearing 9. When the abrasive member 2 attached to the grinding disc 3 is not in contact with a workpiece and the grinding disc 3 is freely rotatable, the rotational speed of the grinding disc 3 about its axis increases to the rotational speed with which the rotary disc 7 is driven. If polishing or grinding is performed by pressing the abrasive member 2 against a surface of the workpiece after the rotational speed of the grinding disc 3 has increased to such a level, the grinding operation is performed impulsively. As a result, marks and scratches are formed on the surface of the workpiece. If the grinding disc 3 is strongly pressed against the workpiece, a brake is applied to the rotation of the grinding disc 3 about its axis, and the braking force becomes larger than a rotational force of the rotary disc 7, which is generated by friction between the rotation shaft 4 and the bearing 9. As a result, the rotation of the grinding disc 3 about its axis is stopped, and therefore the grinding performance is considerably reduced.
In order to prevent such a sharp increase in the rotational speed of the grinding disc about its axis when the grinding disc is unloaded and in order to prevent stopping of the rotation when the grinding disc is pressed against a surface to be ground, brakes and structures for transmitting a driving force for the rotation shaft of the grinding disc have been proposed as described in PTLs 1 to 3. PTL 1 describes a structure with which an increase in the rotational speed of a grinding disc is prevented by friction of braking means, which is an elastic functional ring attached to a casing of a driving motor. When the grinding disc is pressed against a workpiece, the braking means becomes deformed so as to mesh with the grinding disc. Due to such meshing, the grinding disc receives an active driving force from an eccentric member supporter (rotary disc). Therefore, the grinding disc can continue rotating when pressed against the workpiece. However, with this structure, braking for preventing an increase in the rotational speed of the grinding disc about its axis when the grinding disc is unloaded is performed by using friction between the rotation shaft and the elastic functional ring. Such a structure is inefficient because a brake is applied to the grinding disc before the rotational energy of the drive shaft is transmitted to a workpiece, and therefore energy loss is large. Moreover, because a driving force for maintaining a rotational force of the grinding disc is transmitted through the meshing between the braking means attached to the casing and the grinding disc, the grinding disc rotates in a direction opposite to the direction in which the eccentric member supporter (rotary disc) rotates, and the direction of rotation of the grinding disc changes instantaneously during grinding. Therefore, a large shock occurs, and the shock may affect a surface of a workpiece and may cause danger to an operator. Furthermore, the rotational speed of the grinding disc becomes constant relative to that of driving rotation, that is, the grinding disc does not rotate irregularly and smoothly. Therefore, this structure does not provide the function of a grinding tool having an eccentric rotation mechanism.
PTL 2 describes a grinding tool in which a device for limiting the rotational speed of a sanding disc (grinding disc) is attached to a housing (casing), and the device constantly transmits a force to the sanding disc. In the grinding tool, the device for limiting the rotational speed of the sanding disc is a hollow wheel that is connected through a partial bearing to the housing so as not to be rotatable relative to the housing. The connection can be released so that the sanding disc can freely rotate. Therefore, the rotational speed of the sanding disc can be controlled more smoothly than the grinding tool of PTL 1. However, in order to control the rotational speed and the direction of rotation of the sanding disc, it is necessary to perform precise calculations of at least the following: (1) the magnitudes and the directions of a friction moment of a bearing at an engagement portion and a friction moment between a first engagement portion and a second engagement portion; (2) the magnitude and the direction of a friction moment between an eccentric pin and a sanding disc bearing; and (3) the rotational speed and the rotational torque of a drive shaft. Moreover, the grinding tool, in which connection and disconnection of a locking device are performed and a clutch and the like, are used, has a complex mechanism. Furthermore, as in the case of PTL 1, a large shock occurs because the direction of rotation of the sanding disc changes instantaneously during a grinding operation, and the shock may affect a surface of a workpiece or may cause danger to an operator.
The grinding tool described in PTL 3, in which a driving force is directly transmitted to the grinding disc (grinding pad), controls rotation by applying forces in the axial direction to an inner race and an outer race of a bearing. Therefore, the grinding tool has a problem about the durability, which arises due to the structure of the bearing.